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Module 15 Richard > 15.7 Exhausts & Thrust Reversers > Flashcards

15.7 Exhausts & Thrust Reversers Flashcards

1
Q

What is the main purpose of a jet engine exhaust?

A

To accelerate the airflow coming from the turbines to efficiently make thrust

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2
Q

What is the main purpose of the exhaust on a turboprop engine?

A

To safely guide the exhaust gases overboard

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3
Q

What is the purpose of the exhaust cone?

A

It guides the flow and prevents reverse flow of the air.

It also prevents excessive turbulence of the gases.

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4
Q

What type of aircraft use long exhaust ducts?

A

On older aircraft where the engine is mounted directly under the wing

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5
Q

What is the advantage of using a common exhaust nozzle?

A

It mixes the bypass and core air (slowing the velocity of the exhaust gases) which gives a higher propulsion efficiency

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6
Q

What is the disadvantage of using a common exhaust nozzle?

A

Added weight of the engine

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7
Q

What is the advantage of using a mixer on the exhaust of a jet engine?

A

It increases the propulsion efficiency by mixing the air inside a common exhaust nozzle.
They aren’t normally used anymore as they add weight

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8
Q

How is optimum thrust achieved in relation to the exhaust gases?

A

All of the exhaust gases must be fully expanded inside the exhaust nozzle which guarantees a Laminar vortex-free and axial orientated exhaust gas flow

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9
Q

How does the pressure ratio change in relation to the engine speed?

A

Increasing the engine speed increases the pressure ratio (a higher airflow through the exhaust nozzle)

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10
Q

In a convergent exhaust nozzle, what pressure ratio permits maximum airflow?

A

A pressure ratio of 2:1

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11
Q

What is a choked condition of the exhaust nozzle?

A

When the engine speed is increased above the maximum airflow (pressure ratio 2:1) chocking occurs as not all of the air can escape quickly enough.
At the smallest part of the exhaust, the airflow equals the speed of sound.

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12
Q

What are C-D nozzles?

A

Convergent-Divergent nozzles, used mainly on aircraft which fly faster than the speed of sound. They have a variable design

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13
Q

Why must high exhaust gases be avoided on a commercial turbofan or turbojet engine?

A
  • It decreases the propulsion efficiency as it creates turbulent airflow when mixed with the ambient airflow
  • It creates a loud exhaust noise
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14
Q

What is the unit commonly used to express aircraft noise level?

A

Effective Perceived Noise deciBel (EPNdB)

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15
Q

What does a turbofan engine’s noise depend on?

A

The bypass ratio

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16
Q

What is used to provide noise suppression of the internal noise generated?

A

Acoustic linings

17
Q

What is a disadvantage of using acoustic linings to suppress internal noise?

A

Added weight to the engine

18
Q

What is used to suppress the external noise of a low-bypass engine or turbojet engine generated?

A

Hush kits

- lobe type nozzles attached to the exhaust which mixes both core and ambient air (or small amount of bypass air)

19
Q

What is the usual deflection of air when using reverse thrust compared to the ideal?

A

Usual deflection of exhaust gases = 45-60 degrees

Ideal deflection = 180 degrees

20
Q

On a turbofan engine, what air is deflected for reverse thrust?

A

Bypass (fan) air only

21
Q

What are the four different types of thrust reversers?

A
  • Clamshell
  • Bucket-type
  • Translating sleeves (cascade type)
  • Pivot-type
22
Q

How can the cascade type translating sleeve thrust reverser be operated?

A

Pneumatically or Hydraulically

23
Q

What is the modern turbofan engine thrust reverser also known as?

A

Fan reverser

24
Q

What were the disadvantages to the older hot core reversers?

A
  • inefficient

- very extensive maintenance needed due to the hot temperatures

25
Q

What are the disadvantages of using thrust reversers?

A
  • They can whirl up FOD into the intake
  • They are less efficient at low speeds (ingesting own airflow).
  • The compressor can stall due to the deployment of reverse thrust
  • The reversers can cause a cushion of air between the lower wing and the ground, reducing the efficiency of the brakes and landing gear and contact with the runway.
26
Q

How are the disadvantages to thrust reverse minimised?

A

The airflow is matched to engine position and deflected at specific angles for the safest operation. This is completed by the cascade vanes.

27
Q

What must be done to an aircraft with one faulty thrust reverser?

A

It must be deactivated properly so that it does not deploy unintentionally. The aircraft can still be dispatched

28
Q

What are the three main subsystems of a thrust reverser system?

A

1) The control system
2) The actuator system
3) The airflow deflection system

29
Q

How are the thrust reversers protected during flight so that they cannot deploy?

A

The air-ground logic

30
Q

Some aircraft can deploy the thrust reverser immediately prior to landing. How does this system operate?

A

Using the radio altimeter - deployable at less than 10 feet

This is an advantage for short runway landings

31
Q

What is the purpose of the thrust reverser interlock system?

A

It makes sure that the engine power can only be increased after the reverser is fully deployed

32
Q

Why are hydraulic actuators on the translating sleeve type thrust reverser more complicated?

A

They have to be synchronised

33
Q

What type of thrust reverser system could you find a pneumatic actuation system instead of a hydraulic actuation system?

A

Cascade type translating sleeve thrust reversers can be pneumatic or hydraulically operated

34
Q

What type of thrust reverser is usually always pneumatically operated?

A

The older clam-shell thrust reverser (via HP compressor)

35
Q

What is the purpose of the locking system for the thrust reversers?

A

It prevents the thrust reversers deploying unintentionally and them safe in the stowed position.

36
Q

How is a pneumatic thrust reverser actuator locked out?

A

They have brakes at the air motor as the locking device

37
Q

If a cascade vane from the right hand cowling which is an upward pointing segment is placed on the left hand side, where does the vane direct the airflow?

A

If placed on the left hand cowling, it would point downwards

38
Q

What indication would you find in the cockpit that tells the crew that the thrust reverser isn’t safe in its stowed position?

A

A “reverse unlock” message in amber

39
Q

What would a green thrust reverse light mean in the cockpit?

A

That the reverser is fully deployed and the interlock has been released so that you can increase the engine power

Module 15 Richard (18 decks)

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